A brake actuation system of this kind is known, for example, from DE 4,102,497 C1. In this brake actuation system, the buffer memories that can be used for pedal path simulation, which are positioned separately from the master cylinder, are located relatively far from the brake pedal, so that in a control event, no optimum pedal feel can be obtained. Considered to be less favorable is the space requirement associated with the installation of the buffer memory and also the relatively poor ventilation of the buffer memories.
In addition, from DE-OS 4,229,041 A1 an electronically controllable, fail-safe brake actuating system is known in which both the master cylinder and also the electrically controllable brake pressure transducer equipped with a separate hydraulic fluid supply tank can be connected by means of an electromagnetically actuated 3/2-way valve with the wheel brakes. In a first switch setting of the 3/2-way valve, a hydraulic linkage exists between the master cylinder and/or the pressure transducer and the wheel brake, whereas the connection between the pressure transducer and/or the master cylinder and the wheel brake is blocked. In a second switch setting, the 3/2-way valve establishes a hydraulic connection between the pressure transducer and/or the master cylinder and the wheel brake, whereas the connection between the master cylinder and/or the pressure transducer and the wheel brake is blocked. Due to the stated measures, in intact systems the driver is separated from the wheel brake and feels a hard pedal. Since the brake pressure transducers are equipped with separate hydraulic fluid supply tanks, they must be separated from the wheel brake in case of a power failure, so that complicated valves have to be used.
DE-OS 3,424,915 discloses blocking of the hydraulic linkages between the brake pressure transducers and the master cylinder by means of holes (blow holes) drilled in the hydraulic cylinders of the brake pressure transducers, the holes being passed therefor over by sealing sleeves located on their piston upon actuation of the brake pressure transducers.
However, the known brake actuating systems are less well suited for use in antilock brake systems. For example, the locking, hydraulic linkages do not permit the relief of the hydraulic pressure prevailing in the wheel brake down to near zero, in an ABS control event since during the necessary complete restoring motion of the brake pressure transducer piston, the drill hole mentioned above would be released to which the relatively high pressure created by the driver is applied.
Also less advantageous for an ABS control are the chambers formed in the hydraulic cylinders of the brake pressure transducers; the volume of hydraulic fluid displaced by the driver of the vehicle is introduced into said chambers in order to maintain the usual pedal feel for the driver upon actuation. The drive units of the brake pressure transducers must in every case actively ensure the restoring motion of the pressure transducer pistons, so that large costs are required for a four-quadrant control electronics system and also a design for maximum actuating force is necessary. In addition, an undamped pedal restoration takes place against the driver's foot.
Finally, the direct actuation of the valve device formed by the combination of drill hole (blow hole) and sealing sleeve takes place in actuality by means of the electric drive due to the non-negligible mass inertia at a time delay, which, for its part, results in an inflow of a considerable volume of hydraulic fluid, so that a pressure relief to values near zero bar is not possible. This adversely affects particular control processes at lower values.
Therefore it is the purpose of the present invention to improve an electronically controllable brake actuating system of the kind described above in such a manner that in particular the disadvantages stated in an ABS control will be essentially avoided. At the same time, an improvement in pedal feel is to be achieved with simultaneous reduction in the total expense.
This problem is solved according to this invention in that the simulator chamber in the tandem main cylinder is constructed as a constituent of one of its pressure areas, with the simulator piston running in the secondary piston and limiting a hydraulic chamber which is connected to a nonpressurized compressive agent supply tank cooperating with the tandem main cylinder. Due to this activity, a particularly compact design of the invented brake actuation system is achieved.
In a favorable refinement of the object of the invention, the simulator piston can move upon actuation of the tandem main cylinder opposite the actuation direction and cooperates with a stop formed on or in one of the master cylinder pistons. In this case it is particularly favorable if the simulator spring is braced against the master cylinder housing and features a progressive force-path line.
Due to the bracing against the tandem master cylinder housing, the prerequisite is created for a large part of the stroke work stored in the simulator spring to be used again for the brake pressure buildup, when the simulator piston strikes against the master cylinder piston and the simulator spring is relaxed. This working range is intended exclusively for the hydraulic function (failure of the electric system) in order to satisfy the legal requirements for large vehicles.
In another favorable refinement of the object of the invention, the connection between the simulator chamber and the master cylinder can be blocked by means of a second valve device, or the simulator chamber is disconnected from the master cylinder in the nonactuated state of the system. This means that in a failure of the brake pressure transducer, the entire volume of the master cylinder can be used for pressure build up in the wheel brakes, or that the increase in volume of the simulator chamber can be adapted, if necessary, to that of an ordinary brake system.
One aspect of the invention that lends itself to low cost production is that the electromagnetic valves and the second valve devices can be actuated jointly, in particular, that they are designed as 3/2-way valves.
A complete drainage of the simulator chamber, especially in the case of a defective second valve device, is ensured according to an additional property of the invention in that the second valve device or the 3/2-way valves are connected in parallel to the nonreturn valves opening to the master cylinder.
An additional design that lends itself to low cost production, in which a good tightness of the 3/2-way valves in both switch settings (to the simulator chamber and also to the wheel brake) is ensured, consists in designing the 3/2-way valves as double seat valves.
A controlled driving, especially with regard to the threshold values and/or gradients, of the valve devices, which are released only after a fault check, is achieved in an additional embodiment wherein the valve devices can be driven directly by means of the output signals of the sensor unit supplied to the electronic control unit.
A recognition of the driver's deceleration requirements determinable without friction or locking is thus made possible in that the sensor device is formed by a force sensor which determines the actuating force acting on the brake actuating pedal.
A dependable design of the invented brake actuating system manufactured at low cost and in which the sensor device is formed by at least one pressure sensor connected to the master cylinder, consists in designing the pressure sensor as a constituent of the electronic control unit. Since no cable or plug-in connections are used outside of the electronic control unit, the result is a simplification in assembly effort.
A low-friction conversion of a rotational motion into a linear motion, in particular in a favorable refinement of the invention, in which the brake pressure transducers are formed by hydraulic cylinders whose pistons are actuated by means of electrically driven d-c motors, takes place by accomplishing the actuation by means of a ball screw connected between piston and d-c motor. In this case it is useful if the spindle of the ball screw and the shaft of the d-c motor form a structural unit or if the shaft of the d-c motor is of hollow design and the spindle holds the ball screw. Due to these measures, a simple assembly of the brake pressure transducer and/or a simple design of the spindle is attained. In addition, the brake pressure transducers feature a short structural shape.
In order to keep the thermal and electrical or power stress on the d-c motor actuating the brake pressure transducer as small as possible, an additional favorable design of the object of the invention provides that features are provided which allow a reduction in the current supplied to the d-c motor with simultaneous prevention of a decrease in the pressure registered in the wheel brake. Thus the desired brake action is retained, even though the d-c motor cools. The aforementioned features can be formed either by valves located between the wheel brake and the brake pressure transducer or by a friction brake which cooperates with the ball screw of the brake pressure transducer. In the latter design, only a small brake torque is needed.
In this case it is particularly advantageous if the friction brake is electrically controlled. Compared to permanent acting friction brakes, no losses occur during normal operation.
In another favorable refinement of the object of the invention, the d-c motor is reversible. Due to this action, high rates of pressure reduction are also attained even at low brake pressures. In addition, a design of this kind is characterized by low susceptibility to friction effects in a pressure drop and also by a reduced axial length.
In a highly dynamic design in which no problems occur with power demand peaks and in which the brake pressure transducers are formed by hydraulic cylinders whose pistons can be actuated by means of valve-controlled, pneumatic or hydraulically driven linear drives, good control with reduction of the necessary sensor expense is attained by designing the valves used for the control as pressure-regulating valves. The hydraulic control of the linear drive occurs in particular and preferably by means of a single pressure source to which additional hydraulic loads can be attached. Alternatively, the linear drives can be controlled by means of pneumatic high pressure. A design of this kind makes use of the good storability of air and is characterized primarily by a small installation space requirement.
An improvement in controllability is achieved in one refinement of the invention by providing features to ascertain the position of the pressure transducer piston and/or of the angular position of the rotors of the d-c motors.
A particularly good controllability, especially in the lower pressure range, is achieved in an additional design of the object of the invention by integrating the valves used for the control into a position control circuit which processes the output signals from the aforementioned features.
An additional improvement in controllability is achieved according to an additional property of the invention, by providing means for sensing the pressure applied by the pressure transducers, with the valves used for the control being integrated into a pressure control circuit that processes the output signals from the aforementioned means.